1. Field of the Invention
The present invention relates to an optical recording disc, and more particularly, to an optical recording disc comprising a pair of transparent disc substrates, or a transparent disc substrate and a reinforcement substrate.
2. Decription of the Related Art
There is provided an optical recording disc comprising a pair of disc units 25, as shown in FIG. 1, or one disc unit 25 and a reinforcement disc unit 28, as shown in FIG. 2. The recording disc has a transparent disc substrate 21 made of glass, wherein each transparent disc substrate 21 has a center hole 20, a resin layer 23 formed on one surface of the transparent disc substrate 21, and a recording layer 42 formed on the surface of the resin layer 23. In the disc unit 25, a signal pattern 22, being one of pits corresponding to an information signal or guiding grooves corresponding to a tracking signal, is transferred to one surface of the resin layer 23.
In a double sided recording type optical recording disc, as shown in FIG. 1, a pair of the disc units 25 are bonded together through an inner hub 26 and an outer spacer 27 so that the resin layers 23 and the recording layers 24 of both of the disc units 25 face each other. On the other hand, in a single sided recording type optical recording disc, as shown in FIG. 2, the disc unit 25 and the reinforcement disc unit 28 comprising only the transparent disc substrate 21 are bonded together through the inner hub 26 and the outer spacer 27 so that the resin layer 23 and the recording layer 24 face the inside of the optical recording disc.
In the aforementioned optical recording disc, when a laser light is radiated from the upside of the surface of the transparent disc substrate 21 or the reinforcement unit 28, where the resin layer 23 and the recording layer 24 are not formed, onto the boundary surface between the resin layer 23 and the recording layer 24, the reflected light on the boundary surface is inputted and read as the information signal. On the other hand, an information signal is written into the recording layer 24 by radiating a laser light. Accordingly, it is necessary for the transparent disc substrate 21 to have a high transparency, a smooth surface, and a uniformity of thickness, and the laser incident surface of the transparent disc substrate 21 or the reinforcement unit 28 is polished so as to provide the necessary condition.
As shown in FIG. 3, however, the edge portions 29, 30, 31, and 32 of the inner rim portion and the outer rim portion of the transparent disc substrate 21 or the reinforcement unit 28 are not generally processed, since the edge portions 29, 30, 31, and 32 do not directly influence the reading and writing operation of the information signal into and from the optical recording disc. Therefore, rectangular edge portions of the transparent disc substrate 21 or the reinforcement unit 28 are formed without particular processing, such as cutting etc.
As shown in FIG. 4, the inner hub 26 comprises a cylindrical hub portion 26a and a cylindrical inner spacer portion 26b formed in the middle portion of the outer surface of the cylindrical hub portion 26a. The cylindrical inner spacer portion 26b acts as an inner spacer together with the outer spacer 27 when the optical recording disc is fabricated.
The inner hub 26 and the outer spacer 27 are bonded on the surface of the disc unit 25 as follows. First, a proper quantity of bonding material 33 is coated on the outer surface of the cylindrical hub portion 26a and the top and the bottom surfaces of the inner spacer portion 26b of the inner hub 26, respectively facing the transparent disc substrate 21, as shown in FIG. 4, and a proper quantity of the bonding material 33 is coated on the top and the bottom surfaces of the outer spacer 27, respectively facing the transparent disc substrate 21. Then, the transparent disc substrate 21 is pushed to the surfaces of the inner hub 26 and the outer spacer 27, where the bonding material is coated as described above, and the coated bonding material is extended by pushing, resulting in the inner hub 26 and the outer spacer 27 being bonded on the transparent disc substrate 21. In this case, there is no space for accommodating the remainder 33a of the bonding material 33, as shown in FIG. 5, since the rectangular edges 29, 30, and 31 are formed in the inner and outer rim portions of the inside surface of the transparent disc substrate 21, resulting in the remainder 33a of the bonding material 33 being pushed out of the top and the bottom surfaces of the transparent disc substrate 21 and the side surface of the transparent disc substrate 21. It was suggested in Japanese patent laid open No. 185233/1985 that a bonding recess be formed in the inner spacer and/or the outer spacer, however, the bonding recess could not prevent the remainder 33a from pushing out of the top and the bottom surfaces of the transparent disc substrate 21 and the side surface of the transparent disc substrate 21.
In particular, after the remainder 33a of the bonding material 33 is pushed out of the top and the bottom surfaces of the transparent disc substrate 21, this remainder 33a of the bonding material 33 solidifies. When the optical recording disc, with the solidified remainder 33a of the bonding material 33 on the top and the bottom surfaces, is set on a turn table of a recording and play back device as in FIG. 6, the optical recording disc is not horizontally set on the turn table of the recording and play back device, that is, the top and the bottom surfaces of the transparent disc substrate 21 are not parallel to the turn table, resulting in that it is difficult for the laser light radiated from a light head 38 to focus on the boundary surface between the resin layer 23 and the recording layer 24, when the turn table of the recording and play back device is rotating.
In addition, when the optical recording disc with the rectangular edges 29, 30, 31, and 32 of the transparent disc substrate 21 is handled, there is a problem that the rectangular edges 29, 30, 31, and 32 of the transparent disc substrate 21 can not avoid contact with the handle's fingers.